Studies have been completed of a population on the island of Mauke (Cook group, South Pacific) where W. bancrofti filariasis is endemic. Individuals without microfilaremia exhibited strong cellular immune responses to filarial antigens by lymphocyte transformation while those with microfilaremia were poorly responsive. This defect was specific for filarial antigens and did not involve humoral antibody responses. A similar pattern was found in patients with schistosomiasis. Cases studied within three months of initial infection with S. mansoni generated brisk in vitro cellular immune responses to schistosome antigens, whereas lymphocytes from those with chronic infection had a diminished response to the same antigens. Among patients seen in consultation, a case of babesiosis was diagnosed and the organism isolated from an infection acquired on Martha's Vineyard, an island off Massachusetts.